Optical fiber and optical transmission system including the same

ABSTRACT

The present invention relates to an optical fiber which enables favorable optical communications in 1.3-μm and 1.55-μm wavelength bands, and an optical transmission system including the same. The optical fiber according to the present invention has only one zero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60 μm, the zero-dispersion wavelength existing within a wavelength range of 1.37 μm to 1.50 μm, and has a positive dispersion slope at the zero-dispersion wavelength, thereby enabling favorable optical communications utilizing each signal light in the 1.3-μm and 1.55-μm wavelength bands sandwiching the zero-dispersion wavelength.

RELATED APPLICATIONS

This is a Continuation application of U.S. application Ser. No.09/847,438, filed on May 3, 2001, which is a Continuation application ofU.S. application Ser. No. 09/580,483, filed on May 30, 2000, now U.S.Pat. No. 6,266,467, which in turn is a Continuation-In-Part applicationof International Patent Application Serial No. PCT/JP99/06611, filed onNov. 26, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber applicable to atransmission line in optical communications, and an optical transmissionsystem including this optical fiber.

2. Related Background Art

Conventionally, as a transmission line in optical communications,standard single-mode optical fibers having a zero-dispersion wavelengthin a 1.3-μm wavelength band (1280 nm to 1320 nm) have mainly beenutilized. The transmission loss resulting from the main material(silica) of such an optical fiber has been known to become the lowest ina 1.55-μm wavelength band (1530 nm to 1565 nm). In addition, opticalfiber amplifiers using an Er-doped optical fiber can amplify light inthe 1.55-μm wavelength band at a high efficiency. For such a reason,dispersion-shifted optical fibers designed so as to have azero-dispersion wavelength in the 1.55-μm wavelength band are applied totransmission lines in wavelength division multiplexing (WDM)communications for transmitting a plurality of wavelengths of signallight. As for a light source for sending out signal light, devicetechnologies for enabling light in the 1.3-μm wavelength band and lightin the 1.55-μm wavelength band to be outputted have conventionally beenestablished.

SUMMARY OF THE INVENTION

The inventors have studied the prior art mentioned above and, as aresult, found problems as follows. Namely, in the case where light inthe 1.3-μm wavelength band is transmitted while a dispersion-shiftedoptical fiber having a zero-dispersion wavelength in the 1.55-μmwavelength band is used as an optical transmission line, the absolutevalue of dispersion becomes so large that WDM communications cannot becarried out in a wide band. Also, when signal light in the 1.55-μmwavelength band is transmitted through such a dispersion-shifted opticalfiber, the absolute value of dispersion becomes so small that four-wavemixing, which is one of nonlinear optical phenomena, is likely to occur.In the case where light in the 1.3-μm wavelength band is transmittedwhile a standard single-mode optical fiber having a zero-dispersionwavelength in the 1.3-μm wavelength band is used as an opticaltransmission line, on the other hand, the absolute value of dispersionbecomes so small that four-wave mixing, which is one of nonlinearoptical phenomena, is likely to occur. Also, when signal light in the1.55-μm wavelength band is transmitted through such a single-modeoptical fiber, the absolute value of dispersion becomes so large thatWDM communications cannot be carried out in a wide band.

For this matter, attempts have been made to develop optical fibers forsuppressing the occurrence of dispersion over a wide wavelength band(see, for example, K. Okamoto et al., “Zero total in single-mode opticalfibers over an extended spectral range,” Radio Science, Volume 17,Number 1, pages 31-36, January-February 1982). For example, an opticalfiber having a low dispersion value over a wide wavelength band has beenproposed by yielding a large relative refractive index difference of2.4% between its cladding region and core region and a small diameter of3.5 μm in the core region. However, it is difficult to make such anoptical fiber having a very large relative refractive index differencebetween the cladding region and core region, and its transmission lossis large. In an optical fiber whose core region has a smaller diameter,on the other hand, the effective area becomes smaller, and nonlinearoptical phenomena are likely to occur.

In order to overcome problems such as those mentioned above, it is anobject of the present invention to provide an optical fiber whichenables efficient transmission of both of signal light in the1.3-μmwavelength band and signal light in the 1.55-μm wavelength band,and an optical transmission system including the same.

The optical fiber according to the present invention is an optical fiberwhich enables efficient transmission of both of signal light in the1.3-μm wavelength band and signal light in the 1.55-μm wavelength band,the optical fiber having only one zero-dispersion wavelength within awavelength range of 1.20 μm to 1.60 μm and having a positive dispersionslope at the zero-dispersion wavelength. Here, this zero-dispersionwavelength lies within a wavelength range of 1.37 μm to 1.50 μmsandwiched between the 1.3-μm wavelength band and the 1.55-μm wavelengthband. Also, the above-mentioned dispersion slope preferably has anabsolute value of 0.10 ps/nm²/km or less at the above-mentionedzero-dispersion wavelength (preferably 0.06 ps/nm²/km or less at awavelength of 1.55 μm), and monotonously changes (e.g.,monotonouslyincreases) at least in a wavelength range of 1.30 μm to 1.55 μm.

Thus, since this optical fiber has a zero-dispersion wavelength withinthe wavelength range of 1.37 μm to 1.50 μm including a wavelength of1.38 μm at which an increase in transmission loss caused by OHabsorption is seen, dispersion occurs to a certain extent in thevicinity of the 1.3-μm wavelength band and in the vicinity of the1.55-μm wavelength band. As a consequence, the optical fiber comprises astructure in which four-wave mixing is hard to occur even when thesignal light in the 1.3-μm wavelength band and the signal light in the1.55-μm wavelength band propagate therethrough.

In the case where a thulium-doped fiber amplifier having anamplification band in a 1.47-μm wavelength band is utilized, thezero-dispersion wavelength is more preferably set within a wavelengthrange of 1.37 μm to 1.43 μm. It is because of the fact that thetransmission band can further be widened if the zero-dispersionwavelength is aligned with a skirt of the OH absorption peak (1.38 μm).In the case where the above-mentioned OH absorption peak is kept low bydehydration processing or the like, so as to utilize the wavelength bandincluding the wavelength of 1.38 μm as its signal light wavelength band,on the other hand, the zero-dispersion wavelength may be set within awavelength range of longer than 1.45 μm but not longer than 1.50 μm inorder to intentionally generate dispersion in the above-mentionedwavelength band.

In the optical fiber, while the dispersion slope monotonously increases,the absolute value of the dispersion slope at its zero-dispersionwavelength is 0.10 ps/nm²/km or less, and the dispersion slope at awavelength of 1.55 μm is preferably 0.06 ps/nm²/km or less, whereby thedispersion in the 1. 3-μm wavelength band and the dispersion in the1.55-μm wavelength band are homogenized. Here, each of the absolutevalue of dispersion in the 1.3-μm wavelength band and the absolute valueof dispersion in the 1.55-μm wavelength band is 6 ps/nm/km or more but12 ps/nm/km or less.

As mentioned above, the optical fiber according to the present inventionrealizes efficient optical communications in both of the 1.3-μmwavelength band and the 1.55-μm wavelength band. From the viewpoint ofguaranteeing a single mode, the case where the cutoff wavelength is 1.3μm or shorter while the transmission line length is several hundreds ofmeters or less is preferable since only the ground-mode light canpropagate in each of the 1.3-μm wavelength band and the 1.55-μmwavelength band. Also, in view of the dependence of cutoff wavelength ondistance, no practical problem occurs in optical transmission over arelatively long distance (a transmission line length of severalkilometers or less) even if the cutoff wavelength is 1.45 μm or shorter(in the case where it is longer than the signal light wavelength). Fromthe viewpoint of reducing the bending loss, on the other hand, there arecases where the bending loss increases remarkably when the cutoffwavelength is shorter than 1.0 μm. As a consequence, the cutoffwavelength is preferably 1.05 μm or more, more preferably 1.30 μm ormore.

Further, in order to enable efficient optical transmission in the 1.3-μmwavelength band and 1.55-μm wavelength band, the optical fiber accordingto the present invention has a bending loss which becomes 0.5 dB orless, preferably 0.06 dB or less, per turn when wound at a diameter of32 mm at a wavelength of 1.55 μm, and has an effective area A_(eff)which becomes 45 μm² or more, preferably greater than 49 μm² at awavelength of 1.55 μm. Also, the amount of increase in transmission losscaused by OH absorption at a wavelength of 1.38 μm in the optical fiberis 0.1 dB/km or less. In particular, if the amount of increase intransmission loss caused by OH absorption at a wavelength of 1.38 μm is0.1 dB/km or less, then a wavelength band in the vicinity of thiswavelength of 1.38 μm can be utilized for a signal light wavelengthband. In this case, in order to intentionally generate dispersion in thewavelength band in the vicinity of the wavelength of 1.38 μm (in orderto suppress four-wave mixing), the zero-dispersion wavelength may be setwithin a wavelength range of longer than 1.45 μm but not longer than1.50 μm.

Here, the effective area A_(eff) is given, as shown in Japanese PatentApplication Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), by thefollowing expression (1): $\begin{matrix}{A_{eff} = {2{{\pi \left( {\int_{0}^{\infty}{E^{2}r{r}}} \right)}^{2}/\left( {\int_{0}^{\infty}{E^{4}r{r}}} \right)}}} & (1)\end{matrix}$

where E is the electric field accompanying the propagated light, and ris the radial distance from the core center.

The optical fiber according to the present invention has a refractiveindex profile in which the maximum and minimum values of relativerefractive index difference with reference to the refractive index ofpure silica (silica which is not intentionally doped with impurities)are 1% or less and −0.5% or more, respectively. In such a refractiveindex profile, the relative refractive index difference of a highrefractive index region doped with Ge element, for example, with respectto pure silica is 1% or less, whereas the relative refractive indexdifference of a low refractive index region doped with F element, forexample, with respect to pure silica is −0.5% or more, whereby itsmanufacture (refractive index control by doping with impurities) iseasy, and the transmission loss can be lowered. Here, the minimum valueof relative refractive index difference with reference to the refractiveindex of pure silica is preferably −0.2% or more, more preferablygreater than −0.15% from the viewpoint of facilitating the manufactureof the optical fiber.

The optical fiber having various characteristics such as those mentionedabove can be realized by various configurations. Namely, a firstconfiguration of the optical fiber can be realized by a structurecomprising a core region which extends along a predetermined axis andhas a predetermined refractive index, and a cladding region provided onthe outer periphery of the core region. The optical fiber of the firstconfiguration may further comprise a depressed cladding structure. Thedepressed cladding structure is realized when the above-mentionedcladding region is constituted by an inner cladding, provided on theouter periphery of the core region, having a lower refractive index thanthe core region; and an outer cladding, provided on the outer peripheryof the inner cladding, having a refractive index higher than that of theinner cladding but lower than that of the core region.

As with the first configuration, a second configuration of the opticalfiber comprises a core region and a cladding region provided on theouter periphery of the core region. However, the core region isconstituted by a first core having a predetermined refractive index; anda second core, provided on the outer periphery of the first core, havinga lower refractive index than the first core. In the case where theoptical fiber of the second configuration comprises a depressed claddingstructure, the cladding region is constituted by an inner cladding, incontact with the outer periphery of the second core, having a lowerrefractive index than the second core; and an outer cladding, providedon the outer periphery of the inner cladding, having a refractive indexhigher than that of the inner cladding but lower than that of the secondcore.

As with the first configuration, a third configuration of the opticalfiber comprises a core region extending along a predetermined axis and acladding region provided on the outer periphery of the core region. Inparticular, the core region comprises a first core having apredetermined refractive index; a second core, provided on the outerperiphery of the first core, having a lower refractive index than thefirst core; and a third core, provided on the outer periphery of thesecond core, having a higher refractive index than the second core. Inthe case where the optical fiber of the third configuration comprises adepressed cladding structure, the cladding region is constituted by aninner cladding, in contact with the outer periphery of the third core,having a lower refractive index than the third core; and an outercladding, provided on the outer periphery of the inner cladding, havinga refractive index higher than that of the inner cladding but lower thanthat of the third core.

When the third configuration mentioned above is employed, it becomeseasier to obtain an optical fiber having allow dispersion slope of 0.06ps/nm²/km or less at a wavelength of 1.55 μm in particular.

Further, a fourth configuration of the optical fiber also comprises acore region extending along a predetermined axis and a cladding regionprovided on the outer periphery of the core region. In particular, thecore region comprises a first core having a predetermined refractiveindex; a second core, provided on the outer periphery of the first core,having a higher refractive index than the first core. In the case wherethe optical fiber of the fourth configuration comprises a depressedcladding structure, the cladding region is constituted by an innercladding, in contact with the outer periphery of the second-rate, havinga lower refractive index than the second core; and an outer cladding,provided on the outer periphery of the inner cladding, having arefractive index higher than that of the inner cladding but lower thanthat of the second core.

A fifth configuration of the optical fiber comprises a core regionextending along a predetermined axis and a cladding region provided onthe outer periphery of the core region. In particular, the core regioncomprises a first core having a predetermined refractive index; a secondcore, provided on the outer periphery of the first core, having a higherrefractive index than the first core; a third core, provided on theouter periphery of the second core, having a lower refractive index thanthe second core; and a fourth core, provided on the outer periphery ofthe third core, having a higher refractive index than the third core. Inthis fifth mode of optical fiber, the cladding region has a lowerrefractive index than the fourth core.

The optical transmission system according to the present invention isrealized by the optical fiber having such a configuration as thosementioned above. Specifically, the optical transmission system accordingto the present invention comprises, at least, a first transmitter foroutputting first light in the 1.3-μm wavelength band, a secondtransmitter for outputting second light in the 1.55-μm wavelength band,a multiplexer for multiplexing the first light outputted from the firsttransmitter and the second light outputted from the second transmitter,and an optical fiber comprising a configuration mentioned above andhaving one end thereof optically connected to the multiplexer. As aresult of this structure, the optical fiber transmits each of the firstlight and second light multiplexed by the multiplexer. According to theoptical transmission system having such a structure, the first light inthe 1.3-μm wavelength band outputted from the first transmitter is madeincident on the above-mentioned optical fiber by way of the multiplexerand propagates through the optical fiber toward a receiving system. Onthe other hand, the second light in the 1.55-μm wavelength bandoutputted from the second transmitter is made incident on the opticalfiber by way of the multiplexer and propagates through the optical fibertoward the receiving system. Also, as mentioned above, the optical fiberapplied to the optical transmission line comprises a structure enablingefficient optical communications in each of the 1.3-μm wavelength bandand 1.55-μm wavelength band, whereby the optical transmission systemenables large-capacity communications when the optical fiber having sucha special structure is employed therein.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph showing a transmission loss characteristic of anoptical fiber according to the present invention with respect towavelength, whereas

FIG. 1B is a graph showing a dispersion characteristic of the opticalfiber according to the present invention with respect to wavelength;

FIG. 2A is a view showing a cross-sectional structure of first andthirteenth embodiments of the optical fiber according to the presentinvention, whereas

FIG. 2B is a refractive index profile of the optical fiber according tothe first embodiment shown in FIG. 2A;

FIG. 3 is a refractive index profile of an optical fiber according to asecond embodiment;

FIG. 4 is a refractive index profile of optical fibers according tothird, fifteenth, and seventeenth embodiments;

FIG. 5 is a refractive index profile of an optical fiber according to afourth embodiment;

FIG. 6 is a refractive index profile of optical fibers according tofifth, sixteenth, eighteenth, nineteenth, and twenty-first embodiments;

FIG. 7 is a refractive index profile of optical fibers according tosixth, twentieth, and twenty-second embodiments;

FIG. 8 is a refractive index profile of optical fibers according toseventh and eighth embodiments;

FIG. 9 is a refractive index profile of optical fibers according toninth and tenth embodiments;

FIG. 10 is a refractive index profile of optical fibers according toeleventh and twelfth embodiments;

FIG. 11 is a table listing various characteristics of the optical fibersaccording to the first to thirteenth embodiments having variousrefractive index profiles as shown in FIGS. 2B and 3 to 10;

FIG. 12 is a table listing various characteristics of the optical fibersaccording to the fourteenth to twenty-second embodiments;

FIG. 13 is a graph showing a dispersion characteristic of the opticalfiber according to the first embodiment with respect to wavelength;

FIG. 14 is a graph showing a transmission loss characteristic withrespect to wavelength of an optical fiber according to the firstembodiment in which dehydration processing has been insufficient;

FIG. 15 is a graph showing a transmission loss characteristic withrespect to wavelength of optical fibers according to the first andthirteenth embodiments in which dehydration processing has been carriedout sufficiently;

FIG. 16 is a graph showing a transmission loss characteristic withrespect to wavelength of an optical fiber according to the thirteenthembodiment in which dehydration processing has been insufficient;

FIG. 17A is a graph showing relationships between effective area A_(eff)and dispersion slope at a wavelength of 1.55 μm mainly concerning theeighteenth to twenty-second embodiments, whereas

FIG. 17B is a graph showing relationships between cutoff wavelength λcand bending loss per turn when bent at a diameter of 32 mm at awavelength of 1.55 μm concerning main embodiments; and

FIG. 18A is a view showing a schematic configuration of the opticaltransmission system according to the present invention, whereas

FIG. 18B is a view showing a modified example of the opticaltransmission system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the optical fiber and opticaltransmission system according to the present invention will be explainedwith reference to FIGS. 1A to 2B, 3 to 16, and 17A to 18B. Among thedrawings, constituents identical to each other will be referred to withnumerals or letters identical to each other without repeating theiroverlapping explanations.

First, FIG. 1A is a graph showing a transmission loss characteristic ofan optical fiber according to the present invention with respect towavelength, whereas FIG. 1B is a graph showing a dispersioncharacteristic of the optical fiber according to the present inventionwith respect to wavelength.

The optical fiber according to the present invention has only onezero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60μm, whereas this zero-dispersion wavelength exists within a wavelengthrange of 1.37 μm to 1.50 μm. Since a transmission loss due to OHabsorption occurs near a wavelength of 1.38 μm as shown in the graph ofFIG. 1A (see, for example, KAZUHIRO NOGUCHI et al., “Loss Increase forOptical Fibers Exposed to Hydrogen Atmosphere,” JOURNAL OF LIGHTWAVETECHNOLOGY, VOL. LT-3, NO. 2, APRIL 1985), it is not always favorable toapply signal light in the vicinity of this wavelength to opticalcommunications. Therefore, as shown in FIG. 1B, the zero-dispersionwavelength in the optical fiber according to the present invention isset within a wavelength range of 1.37 μm to 1.43 μm including thewavelength of 1.38 μm at which the transmission loss due to OHabsorption occurs, but is kept from being set in the vicinity of the1.3-μm wavelength band and 1.5-μmwavelength band sandwiching thiswavelength band. When the wavelength band including the wavelength of1.38 μm is utilized as a signal light wavelength band, thezero-dispersion wavelength may be set within a range of longer than 1.45μm but not longer than 1.50 μm. Thus, the 1.3-μm wavelength band and1.55-μm wavelength band deviated from a predetermined wavelength bandincluding the zero-dispersion wavelength are utilized as a signalwavelength band in the optical fiber according to the present invention,so that dispersion is intentionally generated in these wavelength bands,while the occurrence of four-wave mixing is effectively suppressed. Whenthe zero-dispersion wavelength is set within the range of 1.37 μm to1.43 μm as mentioned above, the transmission band can further be widenedby use of a thulium-doped fiber amplifier whose amplification band liesin a 1.47-μm wavelength band. In the case where the above-mentioned OHabsorption peak is kept low by dehydration processing or the like, so asto utilize the wavelength band including the wavelength of 1.38 μm as asignal light wavelength band, on the other hand, the zero-dispersionwavelength may be set within a wavelength range of longer than 1.45 μmbut not longer than 1.50 μm in order to intentionally generatedispersion in the above-mentioned wavelength band.

Also, the optical fiber according to the present invention has adispersion with an absolute value of 0.10 ps/nm²/km or less at theabove-mentioned zero-dispersion wavelength (preferably 0.06 ps/nm²/km orless at a wavelength of 1.55 μm), thereby being able to realizehomogenization of the respective dispersions in the 1.3-μm wavelengthband and 1.55-μm wavelength band. Here, in this optical fiber, each ofthe absolute value of dispersion D_(1.3) at a wavelength of 1.3 μm andthe absolute value of dispersion D_(1.55) at a wavelength of 1.55 μm is6 ps/nm/km or more but 12 ps/nm/km or less. Even in view of the factthat a standard single-mode optical fiber having a zero-dispersionwavelength in the 1.3-μm wavelength band has a dispersion value of about17 ps/nm/km in the 1.55-μm wavelength band, the optical fiber accordingto the present invention has a sufficiently small absolute value ofdispersion (12 ps/nm/km or less) in each of the 1.3-μm wavelength bandand 1.55-μm wavelength band, thus being more suitably utilized inoptical communications. Since dispersion occurs to an appropriate extent(6 ps/nm/km or more) in these wavelength bands, on the other hand,four-wave mixing can effectively be kept from occurring.

Further, from the viewpoint of guaranteeing a single mode, the opticalfiber according to the present invention preferably has a cutoffwavelength of 1.3 μm or shorter when its transmission length is notlonger than several hundreds of meters. In this case, only ground-modelight can propagate in each of the 1.3-μm wavelength band and 1.55-μmwavelength band. Also, in view of the dependence of cutoff wavelength ondistance, the cutoff wavelength may be 1.45 μm or shorter in opticaltransmission over a relatively long distance (a transmission line lengthof several kilometers or less). In this specification, the cutoffwavelength is that of LP11 mode measured in a state where an opticalfiber having a length of 2 m is wound by only one turn at a radius of140 mm as defined in a CCITT standard. From the viewpoint of reducingthe bending loss, there are cases where the bending loss remarkablyincreases when the cutoff wavelength is shorter than 1.0 μm. Therefore,the cutoff wavelength is preferably 1.05 μm or more, more preferably1.30 μm or more.

As mentioned above, the optical fiber according to the present inventionis a single-mode optical fiber in which a zero-dispersion wavelength isset within a wavelength range deviated from both of the 1.3-μmwavelength band and 1.55-μm wavelength band, while the dispersion valueis small in each wavelength band, whereby it is suitable as atransmission medium in an optical communication system utilizing aplurality of wavelength bands.

The optical fiber according to the present invention preferably has adispersion slope monotonously changing within a wavelength range of 1.30μm to 1.55 μm (monotonously increasing in the case shown in FIG. 1B).This case is preferable not only in that only one zero-dispersionwavelength can be set within a wavelength range of 1.20 μm to 1.60 μm,but also in that the dispersion in each of the 1.3-μm wavelength bandand 1.55-μmwavelength band would not approach zero (because nonlinearoptical phenomena are likely to occur when the dispersion approacheszero).

The optical fiber according to the present invention preferably has abending loss of 0.5 dB/turn or less, more preferably 0.06 dB/turn orless at a wavelength of 1.55 μm when wound at a diameter of 32 mm. Inthis case, since the bending loss is sufficiently small, the increase inloss caused by cabling and the like can effectively be suppressed. Here,this bending loss (dB/turn) is a value obtained when the transmissionloss of light having a wavelength of 1.55 μm concerning an optical fiberwound about a mandrel having a diameter of 32 mm is converted into aloss value per turn.

In the optical fiber according to the present invention, the effectivearea A_(eff) at a wavelength of 1.55 μm is preferably 45 μm or more,more preferably greater than 49 μm . This value is on a par with orgreater than the effective area in a conventional dispersion-shiftedoptical fiber having a zero-dispersion wavelength in the 1.55-μmwavelength band, so that the optical intensity per unit cross-sectionalarea decreases, whereby the occurrence of nonlinear optical phenomenasuch as four-wave mixing is effectively suppressed.

In the optical fiber according to the present invention, the amount ofincrease α in transmission loss caused by OH absorption at a wavelengthof 1.38 μm is preferably 0.1 dB/km or less. It is because of the factthat the wavelength band applicable to optical communications is widenedthereby, so as to enable larger-capacity optical communications. In thecase where a wavelength band including a wavelength of 1.38 μm isutilized as a signal light wavelength band, the zero-dispersionwavelength is preferably designed to lie within a wavelength range oflonger than 1.45 μm but not longer than 1.50 μm deviated from theabove-mentioned wavelength band, in order to suppress the occurrence ofnonlinear optical phenomena.

Preferably, the optical fiber according to the present invention has arefractive index profile in which the maximum and minimum values ofrelative refractive index difference with reference to the refractiveindex of pure silica (silica which is not intentionally doped withimpurities) are 1% or less and −0.5% or more, respectively. Since therelative refractive index difference of a high refractive index regiondoped with Ge element, for example, with respect to pure silica is 1% orless, whereas the relative refractive index difference of a lowrefractive index region doped with F element, for example, with respectto pure silica is −0.5% or more, an optical transmission medium which isrelatively easy to make and has a low transmission loss is obtained. Forfurther facilitating the manufacture, the minimum value of relativerefractive index difference with reference to the refractive index ofpure silica is preferably −0.2% or more, more preferably greater than−0.15%.

First to twenty-second embodiments of the optical fiber according to thepresent invention will now be explained with reference to FIGS. 2A, 2B,and 3 to 10.

First Embodiment

FIG. 2A is a view showing a cross-sectional structure of an opticalfiber 100 according to the first embodiment, whereas FIG. 2B is arefractive index profile of the optical fiber 100 shown in FIG. 1A. Theoptical fiber 100 according to the first embodiment comprises a coreregion 110, with an outside diameter 2 a, extending along apredetermined axis and having a refractive index n₁; and a claddingregion 120, provided on the outer periphery of the core region 110,having a refractive index n₂ (<n₁). Here, the refractive index of thecore region 110 is higher than that of the cladding region 120. Theoutside diameter 2 a of the core region 110 is 5.2 μm, whereas therelative refractive index difference Δ₁ of the core region 110 withreference to the cladding region 120 is 0.55%. Such an optical fiber isobtained when, while silica is used as a base, the core region 110 isdoped with Ge element, for example.

The abscissa of the refractive index profile 150 shown in FIG. 2Bcorresponds to individual parts, along the line L in FIG. 2A, on a crosssection perpendicular to the center axis of the core region 110. Hence,in the refractive index profile 150 of FIG. 2B, areas 151 and 152indicate the refractive indices in individual parts on the line L of thecore region 110 and the cladding region 120, respectively.

Here, the relative refractive index difference Δ₁ of the core region 110with respect to the outermost cladding region 120 is defined as follows:

Δ₁=(n ₁ −n ₂)/n ₂

where n₁ is the refractive index of the core region 110, and n₂ is therefractive index of the cladding region 120. Also, in thisspecification, the relative refractive index difference Δ is expressedin terms of percentage, and the respective refractive indices ofindividual regions in the above-mentioned defining expression may bearranged in any order. Consequently, a negative value of Δ indicatesthat the refractive index of its corresponding region is lower than thatof the cladding region 120.

The optical fiber according to the first embodiment has azero-dispersion wavelength at 1.44 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.060 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.054 ps/nm²/km, andthe cutoff wavelength is 0.96 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−18.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −9.6ps/nm/km, the dispersion at a wavelength of 1.45 μm is 0.6 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 6.2 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.8 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.06 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 49.1 μm².

Second Embodiment

FIG. 3 is a refractive index profile of an optical fiber according tothe second embodiment. The basic configuration of the optical fiberaccording to the second embodiment is similar to that of the firstembodiment shown in FIG. 2A, but is different therefrom in that thecladding region 120 shown in FIG. 2A is modified to have a depressedcladding structure. Referring to FIG. 2A for explanation, the opticalfiber according to the second embodiment comprises a core region 110with an outside diameter 2 a having a refractive index n₁, and acladding region 120 provided on the outer periphery of the core region110. The cladding region 120 is constituted by an inner cladding with anoutside diameter 2 b, provided in contact with the core region 110,having a refractive index n₃ (<n₁); and an outer cladding, provided onthe outer periphery of the inner cladding, having a refractive index n₂(<n₁,>n₃). Here, the outside diameter 2 a of the core region 110 is 5.2μm, whereas the outside diameter 2 b of the inner cladding region is10.9 μm. Also, with reference to the refractive index n₂ of the outercladding region, the relative refractive index difference Δ₁(=(n₁−n₂)/n₂) of the core region is 0.55%, whereas the relativerefractive index difference Δ₂ (=(n₃−n₂)/n₂) of the inner cladding is−0.05%. Such an optical fiber is obtained when, for example, whilesilica is used as a base, the core region and the inner cladding aredoped with Ge element and F element, respectively.

As for the relationship between the refractive index profile 250 shownin FIG. 3 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 250 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 250 of FIG. 3, areas 251, 252, and 253 indicate the refractiveindices in individual parts on the line L of the core region 110, theinner cladding constituting the cladding region 120, and the outercladding constituting the cladding region 120, respectively.

The optical fiber according to the second embodiment has azero-dispersion wavelength at 1.46 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.053 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.049 ps/nm²/km, andthe cutoff wavelength is 0.93 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−18.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.1ps/nm/km, the dispersion at a wavelength of 1.45 μm is −0.5 ps/nm/km,the dispersion at a wavelength of 1.55 μm is 4.3 ps /nm/km, and thedispersion at a wavelength of 1.60 μm is 6.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.20 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 47.2 μm².

Third Embodiment

FIG. 4 is a refractive index profile of an optical fiber according tothe third embodiment. The basic configuration of the optical fiberaccording to the third embodiment is also similar to that of the firstembodiment shown in FIG. 2A, but is different therefrom in that the coreregion 110 shown in FIG. 2A is constituted by a first core and a secondcore. Referring to FIG. 2A for explaining the configuration of theoptical fiber according to the third embodiment, the core region 110comprises a first core, with an outside diameter 2 a, having a maximumrefractive index n₁ at the optical axis center; and a second core withan outside diameter 2 b, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁). The cladding region 120provided on the outer periphery of the second core has a refractiveindex n₃ (<n₂).

As for the relationship between the refractive index profile 350 shownin FIG. 4 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 350 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 350 of FIG. 4, areas 351, 352, and 353 indicate the refractiveindices in individual parts on the line L of the first core constitutingthe core region 110, the second core constituting the core region 110,and the cladding region 120, respectively. Here, the outside diameter 2a of the first core constituting the core region 110 is 6.4 μm, whereasthe outside diameter 2 b of the second core region is 16.0 μm. Withreference to the refractive index n₃ of the cladding region 120, therelative refractive index difference Δ₁ (=(n₁−n₃)/n₃) of the first coreis 0.60%, whereas the relative refractive index difference Δ₂(=(n₂−n₃)/n₃) of the second core is 0.10%. Such an optical fiber isobtained when, for example, while silica is used as a base, the firstcore and the second core are doped with their respective appropriateamounts of Ge element.

The optical fiber according to the third embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.079 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.070 ps/nm²/km, andthe cutoff wavelength is 1.19 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.6ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.1 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.3 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.8 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.006 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 63.6 μm².

Fourth Embodiment

FIG. 5 is a refractive index profile of an optical fiber according tothe fourth embodiment. As in the first embodiment shown in FIG. 2A, theoptical fiber according to the fourth embodiment comprises a core region110 and a cladding region 120. However, it differs from theabove-mentioned third embodiment in that the cladding region 120comprises a depressed structure. Referring to FIG. 2A for explaining theconfiguration of the optical fiber according to the fourth embodiment,as in the third embodiment, the core region 110 comprises a first core,with an outside diameter 2 a, having a maximum refractive index n₁ atthe optical axis center; and a second core with an outside diameter 2 b,provided on the outer periphery of the first core, having a refractiveindex n₂ (<n₁). The cladding region 120 comprises an inner cladding withan outside diameter 2 c, provided in contact with the outer periphery ofthe second core, having a refractive index n₄ (<n₂); and an outercladding, provided on the outer periphery of the inner cladding, havinga refractive index n₃ (>n₄, <n₂).

As for the relationship between the refractive index profile 450 shownin FIG. 5 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 450 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 450 of FIG. 5, areas 451, 452, 453, and 454 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the inner cladding constituting the cladding region 120, andthe outer cladding constituting the cladding region 120, respectively.Here, the outside diameter 2 a of the first core constituting the coreregion 110 is 6.3 μm, the outside diameter 2 b of the second core regionis 16.1 μm, and the outside diameter 2 c of the inner cladding is 28.8μm. With reference to the refractive index of pure silica, the relativerefractive index difference Δ₁ (=(n₁−n₃)/n₃) of the first core is 0.60%,the relative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of the secondcore is 0.10%, and the relative refractive index difference Δ₄(=(n₄−n₃)/n₃) of the inner cladding is −0.05%. Such an optical fiber isobtained when, for example, while silica is used as a base, the firstcore and the second core are doped with their respective appropriateamounts of Ge element, whereas the inner cladding is doped with Felement.

The optical fiber according to the fourth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.081 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.070 ps/nm²/km, andthe cutoff wavelength is 1.15 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.3 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −9.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.1 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 10.2 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 13.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.004 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 62.0 μm².

Fifth Embodiment

FIG. 6 is a refractive index profile of an optical fiber according tothe fifth embodiment. The basic configuration of the optical fiberaccording to the fifth embodiment is also similar to the firstembodiment shown in FIG. 2A, and is constituted by a core region 110 anda cladding region 120. As for the configuration of the optical fiberaccording to the fifth embodiment shown in FIG. 2A, the core region 110comprises a first core, with an outside diameter 2 a, extending along apredetermined axis and having a refractive index n₁; a second core withan outside diameter 2 b, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁); and a third core with anoutside diameter 2 c, provided on the outer periphery of the secondcore, having a refractive index n₃ (>n₂, <n₁). The cladding region 120provided on the outer periphery of the third core 3 has a refractiveindex n₄ (<n₁, <n₃).

As for the relationship between the refractive index profile 550 shownin FIG. 6 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 550 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 550 of FIG. 6, areas 551, 552, 553, and 554 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the third core constituting the core region 110, and thecladding region 120, respectively. Here, the outside diameter 2 a of thefirst core is 5.3 μm, the outside diameter 2 b of the second core regionis 10.0 μm, and the outside diameter 2 c of the third core region is16.6 μm. With reference to the refractive index of the cladding region,the relative refractive index difference Δ₁ (=(n₁−n₄)/n₄) of the firstcore is 0.58%, the relative refractive index difference of the secondcore is 0% since it is set such that n₂=n₄, and the relative refractiveindex difference Δ₃ (=(n₃−n₄)/n₄) of the third core is 0.14%. Such anoptical fiber is obtained when, for example, while silica is used as abase, the first core and the third core are doped with their respectiveappropriate amounts of Ge element.

The optical fiber according to the fifth embodiment has azero-dispersion wavelength at 1.48 μm, and only this one zero-dispersionwavelength exists within a wavelength grange of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.064 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.064 ps/nm²/km, andthe cutoff wavelength is 1.24 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.3 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −11.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is −1.9 ps/nm/km,the dispersion at a wavelength of 1.55 μm is 4.8 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.0 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.008 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 53.9 μm².

Sixth Embodiment FIG. 7 is a refractive index profile of an opticalfiber according to the sixth embodiment. As in the first embodimentshown in FIG. 2A, the basic configuration of the optical fiber accordingto the sixth embodiment comprises a core region 110 and a claddingregion 120. However, it differs from the fifth embodiment in that thecladding region 120 comprises a depressed cladding structure. Referringto FIG. 2A for explaining the configuration of the optical fiberaccording to the sixth embodiment, the core region 110 comprises a firstcore, with an outside diameter 2 a, extending along a predetermined axisand having a refractive index n₁; a second core with an outside diameter2 b, provided on the outer periphery of the first core, having arefractive index n₂ (<n₁); and a third core with an outside diameter 2c, provided on the outer periphery of the second core, having arefractive index n₃ (<n₁, >n₂). The cladding region 120 of depressedcladding structure comprises an inner cladding with an outside diameter2 d, provided on the outer periphery of the third core, having arefractive index n₅ (<n₃); and an outer cladding, provided on the outerperiphery of the inner cladding, having a refractive index n₄ (<n₃,>n₅).

As for the relationship between the refractive index profile 650 shownin FIG. 7 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 650 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 650 of FIG. 7, areas 651, 652, 653, 654, and 655 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the third core constituting the core region 110, the innercladding constituting the cladding region 120, and the outer claddingconstituting the cladding region 120, respectively. Here, the outsidediameter 2 a of the first core is 5.7 μm, the outside diameter 2 b ofthe second core is 16.2 μm, the outside diameter 2 c of the third coreregion is 23.0 μm, and the outside diameter 2 d of the inner cladding 2d is 34.4 μm. With reference to the refractive index of the outercladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.50%, the relative refractive indexdifference of the second core is 0% since it is set such that n₂ =n₄,the relative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the thirdcore is 0.16%, and the relative refractive index difference Δ₅(=(n₅−n₄)/n₄) of the inner cladding is −0.10%. Such an optical fiber isobtained when, for example, while silica is used as a base, the firstcore and the third core are doped with their respective appropriateamounts of Ge element, whereas the inner cladding is doped with Felement.

The optical fiber according to the sixth embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.056 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.052 ps/nm²/km, andthe cutoff wavelength is 1.23 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−16.4 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is 1.6 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 6.6 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.2 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.02 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 57.1 μm².

Seventh and Eighth Embodiments

FIG. 8 is a refractive index profile of optical fibers according to theseventh and eighth embodiments. Both of the seventh and eighthembodiments have the same configuration, each comprising a core region110 and a cladding region 120 as with the first embodiment shown in FIG.2A. Referring to FIG. 2A for explaining the configuration of the opticalfibers according to the seventh and eighth embodiments, the core region110 comprises a first core, with an outside diameter 2 a, extendingalong a predetermined axis and having a refractive index n₁; and asecond core with an outside diameter 2 b, provided on the outerperiphery of the first core, having a refractive index n₂ (>n₁). Thecladding region 120, provided on the outer periphery of the second core,has a refractive index n₃ (<n₂) .

As for the relationship between the refractive index profile 750 shownin FIG. 8 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 750 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 750 of FIG. 8, areas 751, 752, and 753 indicate the refractiveindices in individual parts on the line L of the first core constitutingthe core region 110, the second core constituting the core region 110,and the cladding region 120, respectively.

In the optical fiber according to the seventh embodiment, the outsidediameter 2 a of the first core is 2.8 μm, whereas the outside diameter 2b of the second core is 5.6 μm. With reference to the refractive indexof the cladding region, the relative refractive index difference Δ₁ ofthe first core is 0% since it is set such that n₁=n₃, whereas therelative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of the second coreis 0.7%. Such an optical fiber is obtained when, for example, whilesilica is used as a base, the second core is doped with Ge element.

The optical fiber according to the seventh embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.075 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.061 ps/nm²/km, andthe cutoff wavelength is 1.10 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.1 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −9.3ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.0 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.4 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.4 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.3 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 67.3 μm².

In the optical fiber according to the eighth embodiment, on the otherhand, the outside diameter 2 a of the first core is 3.2 μm, whereas theoutside diameter 2 b of the second core is 6.4 μm. With reference to therefractive index of the cladding region, the relative refractive indexdifference Δ₁ (=(n₁−n₃)/n₃) of the first core is −0.2%, whereas therelative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of the second coreis 0.7%. Such an optical fiber is obtained when, for example, whilesilica is used as a base, the first core and the second core are dopedwith F element and Ge element, respectively.

The optical fiber according to the eighth embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.084 ps/nm²/km,the dispersion slope at a wavelength of1.55 μm is 0.068 ps/nm²/km, andthe cutoff wavelength is 1.17 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−22.9 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −11.1ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.4 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.9 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 13.2 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.2 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 79.1 μm².

Ninth and Tenth Embodiments

FIG. 9 is a refractive index profile of optical fibers according to theninth and tenth embodiments. Both of the ninth and tenth embodimentshave the same configuration, each comprising a core region 110 and acladding region 120 as with the first embodiment shown in FIG. 2A.However, the ninth and tenth embodiments differ from the seventh andeighth embodiments in that the cladding region 120 comprises a depressedcladding structure. Referring to FIG. 2A for explaining theconfiguration of the optical fibers according to the ninth and tenthembodiments, the core region 110 comprises a first core, with an outsidediameter 2 a, extending along a predetermined axis and having arefractive index n₁; and a second core with an outside diameter 2 b,provided on the outer periphery of the first core, having a refractiveindex n₂ (>n₁). The cladding region with the depressed claddingstructure comprises an inner cladding with an outside diameter 2 c,provided on the outer periphery of the second core, having a refractiveindex n₄ (<n₁); and an outside cladding, provided on the outer peripheryof the inner cladding, having a refractive index n₃ (>n₄).

As for the relationship between the refractive index profile 850 shownin FIG. 9 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 850 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 850 of FIG. 9, areas 851, 852, 853, and 854 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the inner cladding constituting the cladding region 120, andthe outer cladding constituting the cladding region 120, respectively.

In the optical fiber according to the ninth embodiment, the outsidediameter 2 a of the first core is 3.8 μm, the outside diameter 2 b ofthe second core is 7.1 μm, and the outside diameter 2 c of the innercladding is 10.6 μm. With reference to the refractive index of the outercladding, the relative refractive index difference Δ₁ of the first coreis 0% since it is set such that n₁=n₃, the relative refractive indexdifference Δ₂ (=(n₂−n₃)/n₃) of the second core is 0.7%, and the relativerefractive index difference Δ₄ (=(n₄−n₃)/n₃) of the inner cladding is−0.2%. Such an optical fiber is obtained when, for example, while silicais used as a base, the second core and the inner cladding are doped withGe element and F element, respectively.

The optical fiber according to the ninth embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.077 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.061 ps/nm²/km, andthe cutoff wavelength is 1.22 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.6 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.2ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.2 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.1 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.1 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.2 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 73.5 μm².

In the optical fiber according to the tenth embodiment, on the otherhand, the outside diameter 2 a of the first core is 2.6 μm, the outsidediameter 2 b of the second core is 6.4 μm, and the outside diameter 2 cof the inner cladding is 9.6 μm. With reference to the refractive indexof the outer cladding, the relative refractive index difference Δ₁(=(n₁−n₃)/n₃) of the first core is −0.2%, the relative refractive indexdifference Δ₂ (=(n₂−n₃)/n₃) of the second core is 0.7%, and the relativerefractive index difference Δ₄ (=(n₄−n₃)/n₃) of the inner cladding is−0.2%. Such an optical fiber is obtained when, for example, while silicais used as a base, the second core is doped with Ge element, whereas thefirst core and the inner cladding are each doped with F element.

The optical fiber according to the tenth embodiment has azero-dispersion wavelength at 1.44 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.070 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.058 ps/nm²/km, andthe cutoff wavelength is 1.18 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.8ps/nm/km, the dispersion at a wavelength of 1.45 μm is 0.7 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.3 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 10.1 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.03 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 59.6 μm².

Eleventh and Twelfth Embodiments

FIG. 10 is a refractive index profile of optical fibers according to theeleventh and twelfth embodiments. Both of the eleventh and twelfthembodiments have the same configuration, each comprising a core region110 and a cladding region 120 as with the first embodiment shown in FIG.2A. Referring to FIG. 2A for explaining the configuration of the opticalfibers according to the eleventh and twelfth embodiments, the coreregion 110 comprises a first core, with an outside diameter 2 a,extending along a predetermined axis and having a refractive index n₁; asecond core with an outside diameter 2 b, provided on the outerperiphery of the first core, having a refractive index n₂ (>n₁); a thirdcore with an outside diameter 2 c, provided on the outer periphery ofthe second core, having a refractive index n₃ (<n₂); and, a fourth corewith an outside diameter 2 d, provided on the outer periphery of thethird core, having a refractive index n₄ (<n₂, >n₃). The cladding region120 provided on the outer periphery of the fourth core has a refractiveindex n₅ (<n₄).

As for the relationship between the refractive index profile 950 shownin FIG. 10 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 950 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 950 of FIG. 10, areas 951, 952, 953, 954, and 955 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the third core constituting the core region 110, the fourthcore constituting the core region 110, and the cladding region 120,respectively.

In the optical fiber according to the eleventh embodiment, the outsidediameter 2 a of the first core is 2.7 μm, the outside diameter 2 b ofthe second core is 5.4 μm, the outside diameter 2 b of the third core is8.1 μm, and the outside diameter 2 d of the fourth core is 10.8 μm. Withreference to the refractive index of the cladding region, the relativerefractive index difference Δ₁ of the first core is 0% since it is setsuch that n, =n₃, the relative refractive index difference Δ₂ (=(n₂−n₅)/n₅) of the second core is 0.8%, the relative refractive indexdifference Δ₃ of the third core is 0% since it is set such that n₃=n₅,and the relative refractive index difference Δ₄ (=(n₄−n₅) /n₅) of thefourth core is 0.1%. Such an optical fiber is obtained when, forexample, while silica is used as a base, the second core and the fourthcore are each doped with Ge.

The optical fiber according to the eleventh embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.080 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.065 ps/nm²/km, andthe cutoff wavelength is 1.16 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.5ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.3 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.5 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.005 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 62.6 μm².

In the optical fiber according to the twelfth embodiment, on the otherhand, the outside diameter 2 a of the first core is 3.2 μm, the outsidediameter 2 b of the second core is 7.0 μm, the outside diameter 2 c ofthe third core is 9.0 μm, and the outside diameter 2 d of the fourthcore 2 d is 12.8 μm. With reference to the refractive index of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₅)/n₅) of the first core is −0.2%, the relative refractive indexdifference Δ₂ (=(n₂−n₅) /n₅) of the second core is 0.6%, the relativerefractive index difference Δ₃ (=(n₃−n₅)/n₅) of the third core is −0.2%,and the relative refractive index difference Δ₄ (=(n₄−n₅)/n₅) of thefourth core is 0.1%. Such an optical fiber is obtained when, forexample, while silica is used as a base, the second core and the fourthcore are each doped with Ge element, whereas the first core and thethird core are each doped with F element.

The optical fiber according to the twelfth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.088 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.071 ps/nm²/km, andthe cutoff wavelength is 1.22 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−22.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.6ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.4 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 11.0 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 14.5 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.4 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 92.7 μm².

Thirteenth Embodiment

The thirteenth embodiment of the optical fiber according to the presentinvention has a cross-sectional structure and a refractive index profilesuch as those basically shown in FIGS. 2A and 2B. However, the opticalfiber according to the thirteenth embodiment differs from the firstembodiment in that, while silica is used as a base, the cladding region120 excluding the core region 110 is doped with fluorine (refractiveindex lowering agent), so as to generate a relative refractive indexdifference between the core region 110 and the cladding region 120.

In the optical fiber according to the thirteenth embodiment, the coreregion 110 made of pure silica (with a refractive index n₀) has anoutside diameter 2 a of 5.6 μm. With reference to the refractive indexn₂ (<n₀) of the cladding region 120, the relative refractive indexdifference Δ₁ (=(n₀−n₂)/n₂) of the core region 110 is 0.53%. Also,though the core region 110 is constituted by pure silica (silica whichis not intentionally doped with impurities) in the thirteenthembodiment, it may be made of silica doped with chlorine.

The optical fiber according to the thirteenth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.057 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.048 ps/nm²/km, andthe cutoff wavelength is 1.04 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−15.7 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.2ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.2 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.1 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.4 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.04 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 52.2 μm².

In addition, the optical fiber according to the thirteenth embodimentyields a transmission loss of 0.17 dB/km at a wavelength of 1.55 μm,thus being an optical fiber of a lower transmission loss as comparedwith the embodiments whose core region is doped with Ge (yielding atransmission loss of about 0.20 dB/km at the wavelength of 1.55 μm).

Fourteenth Embodiment

The fourteenth embodiment of the optical fiber according to the presentinvention has a refractive index profile similar to that of the thirdembodiment shown in FIG. 4, while having an effective area of about 50μm² at a wavelength of 1.55 μm. However, the profile form of thefourteenth embodiment differs from that of the third embodiment in thatthe refractive index (n₁) of the first core is radially uniform.

As in the above-mentioned third embodiment, the optical fiber accordingto the fourteenth embodiment comprises a first core having a refractiveindex n₁; a second core, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁); and a cladding region,provided on the outer periphery of the second core, having a refractiveindex n₃ (<n₂).

In the optical fiber according to the fourteenth embodiment, the outsidediameter 2 a of the first core is 5.5 μm, whereas the outside diameter 2b of the second core is 23.7 μm. With reference to the refractive indexn₃ of the cladding region, the relative refractive index difference Δ₁(=(n₁−n₃)/n₃) of the first core is 0.59%, whereas the relativerefractive index difference Δ₂ (=(n₂−n₃)/n₃) of the second core is0.06%.

The optical fiber according to the fourteenth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.065 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.055 ps/nm²/km, andthe cutoff wavelength is 1.25 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−16.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.7ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.5 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 8.5 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 11.2 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.00002 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 50.1 μm².

Fifteenth Embodiment

The fifteenth embodiment of the optical fiber according to the presentinvention also has a refractive index profile similar to that of thethird embodiment shown in FIG. 4, while having a zero-dispersionwavelength in the vicinity of 1450 nm. However, the profile form of thefifteenth embodiment differs from that of the third embodiment in thatthe refractive index (n₁) of the first core is radially uniform.

As in the above-mentioned third embodiment, the optical fiber accordingto the fifteenth embodiment comprises a first core having a refractiveindex n₁; a second core, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁); and a cladding region,provided on the outer periphery of the second core, having a refractiveindex n₃ (<n₂).

In the optical fiber according to the fifteenth embodiment, the outsidediameter 2 a of the first core is 4.8 μm, whereas the outside diameter 2b of the second core is 15.1 μm. With reference to the refractive indexn₃ of the cladding region, the relative refractive index difference Δ₁(=(n₁−n₃)/n₃) of the first core is 0.65%, whereas the relativerefractive index difference Δ₂ (=(n₂−n₃)/n₃) of the second core is0.06%.

The optical fiber according to the fifteenth embodiment has azero-dispersion wavelength at 1.46 (1.457) μm, and only this onezero-dispersion wavelength exists within a wavelength range of 1.20 μmto 1.60 μm. The dispersion slope at the zero-dispersion wavelength is0.060 ps/nm²/km, the dispersion slope at a wavelength of 1.55 μm is0.060 ps/nm²/km, and the cutoff wavelength is 1.07 μm. Also, thedispersion slope monotonously increases at least in a wavelength rangeof 1.30 μm to 1.55 μm; and, specifically, the dispersion at a wavelengthof 1.20 μm is −20.2 ps/nm/km, the dispersion at a wavelength of 1.30 μmis −11.1 ps/nm/km, the dispersion at a wavelength of 1.45 μm is −0.6ps/nm/km, the dispersion at a wavelength of 1.55 μm is 5.7 ps/nm/km, andthe dispersion at a wavelength of 1.60 μm is 8.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.00006 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 45.3 μm².

Sixteenth Embodiment

The sixteenth embodiment of the optical fiber according to the presentinvention has a refractive index profile similar to that of the fifthembodiment shown in FIG. 6, but differs therefrom in that the refractiveindex (n₂) of the second core is set higher than the refractive index(n₄) of the cladding region and in that the form of the refractive indexprofile of the first core is an α-type distribution (dome form such asone shown in FIG. 5).

As in the above-mentioned fifth embodiment, the optical fiber accordingto the sixteenth embodiment comprises a first core having a maximumrefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); a third core,provided on the outer periphery of the second core, having a refractiveindex n₃ (>n₂, <n₁); and a cladding region, provided on the outerperiphery of the third core, having a refractive index n₄ (<n₃).

In the optical fiber according to the sixteenth embodiment, the outsidediameter 2 a of the first core is 6.7 μm, the outside diameter 2 b ofthe second core is 13.4 μm, and the outside diameter 2 c of the thirdcore is 22.4 μm. With reference to the refractive index n₄ of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.60%, the relative refractive indexdifference Δ₂ (=(n₂−n₄)/n₄) of the second core is 0.05%, and therelative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the third coreis 0.11%.

The optical fiber according to the sixteenth embodiment has azero-dispersion wavelength at 1.47 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.065 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.065 ps/nm²/km, andthe cutoff wavelength is 1.37 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.1 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −12.1ps/nm/km, the dispersion at a wavelength of 1.45 μm is −1.3 ps/nm/km,the dispersion at a wavelength of 1.55 μm is 5.1 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.4 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.02 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 62.6 μm².

Seventeenth Embodiment

The seventeenth embodiment of the optical fiber according to the presentinvention has a refractive index profile similar to that of the thirdembodiment shown in FIG. 4, while having a cutoff wavelength longer thanits signal light wavelength.

As in the above-mentioned third embodiment, the optical fiber accordingto the seventeenth embodiment comprises a first core having a refractiveindex n₁; a second core, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁); and a cladding region,provided on the outer periphery of the second core, having a refractiveindex n₃ (<n₂).

In the optical fiber according to the seventeenth embodiment, theoutside diameter 2 a of the first core is 7.5 μm, whereas the outsidediameter 2 b of the second core is 29.0 μm. With reference to therefractive index n₃ of the cladding region, the relative refractiveindex difference Δ₁ (=(n₁−n₃)/n₃) of the first core is 0.61%, whereasthe relative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of the secondcore is 0.10%.

The optical fiber according to the seventeenth embodiment has azero-dispersion wavelength at 1.40 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.071 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.059 ps/nm²/km, andthe cutoff wavelength is 1.78 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−17.4 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.7ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.5 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.7 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.6 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.00002 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 60.3 μm².

As for the optical fiber having a triple structure in which the coreregion is constituted by the first to third cores as shown in FIGS. 6and 7, a plurality of embodiments having such a low dispersion slopethat the dispersion at a wavelength of 1.55 μm is 0.06 ps/nm²/km or lesswill now be explained.

Eighteenth Embodiment

The eighteenth embodiment of the optical fiber according to the presentinvention has a refractive index profile similar to that of the fifthembodiment shown in FIG. 6, while having a low dispersion slope.

As in the above-mentioned fifth embodiment, the optical fiber accordingto the eighteenth embodiment comprises a first core having a refractiveindex n₁; a second core, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁); a third core, provided on theouter periphery of the second core, having a refractive index n₃ (>n₂,<n₁); and a cladding region, provided on the outer periphery of thethird core, having a refractive index n₄ (=n₂).

In the optical fiber according to the eighteenth embodiment, the outsidediameter 2 a of the first core is 5.5 μm, the outside diameter 2 b ofthe second core is 22.8 μm, and the outside diameter 2 c of the thirdcore is 34.6 μm. With reference to the refractive index n₄of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.48%, the relative refractive indexdifference of the second core is 0% since it is set such that n₂=n₄, andthe relative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the thirdcore is 0.12%.

The optical fiber according to the eighteenth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.058 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.040 ps/nm²/km, andthe cutoff wavelength is 1.75 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−16.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.5ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.1 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 6.8 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.6 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.2 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 57.1 μm².

Nineteenth Embodiment

The nineteenth embodiment of the optical fiber according to the presentinvention is also an optical fiber having a refractive index profilesimilar to that of the fifth embodiment shown in FIG. 6, while having alow dispersion slope. The refractive index profile of the nineteenthembodiment differs from that of the fifth embodiment or that of theabove-mentioned eighteenth embodiment in that the refractive index (n₂)of the second core is set higher than the refractive index (n₄) of thecladding region.

As in the above-mentioned fifth embodiment, the optical fiber accordingto the nineteenth embodiment comprises a first core having a refractiveindex n₁; a second core, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁); a third core, provided on theouter periphery of the second core, having a refractive index n₃ (>n₂,<n₁); and a cladding region, provided on the outer periphery of thethird core, having a refractive index n₄ (<n₃).

In the optical fiber according to the nineteenth embodiment, the outsidediameter 2 a of the first core is 6.2 μm, the outside diameter 2 b ofthe second core is 19.9 μm, and the outside diameter 2 c of the thirdcore is 28.4 μm. With reference to the refractive index n₄ of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.44%, the relative refractive indexdifference Δ₂ (=(n₂−n₄ )/n₄) of the second core is 0.01%, and therelative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the third coreis 0.13%.

The optical fiber according to the nineteenth embodiment has azero-dispersion wavelength at 1.38 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.065 ps/nm²/km,the dispersion slope at a wavelength of1.55 μm is 0.047 ps/nm²/km, andthe cutoff wavelength is 1.52 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−14.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −5.4ps/nm/km, the dispersion at a wavelength of 1.45 μm is 4.4 ps/nm/km, thedispersion at a wavelength of 1.355 μm is 9.4 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 11.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.07 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 64.5 μm².

Twentieth Embodiment

As with the sixth embodiment shown in FIG. 7, the twentieth embodimentof the optical fiber according to the present invention is an opticalfiber having a refractive index profile of a depressed claddingstructure, while having a low dispersion slope. In the refractive indexprofile of the twentieth embodiment, as in the above-mentionednineteenth embodiment, the refractive index (n₂) of the second core isset higher than the refractive index (n₄) of the cladding region.

In the optical fiber according to the twentieth embodiment, as in theabove-mentioned sixth embodiment, the core region comprises a first corehaving a refractive index n₁; a second core, provided on the outerperiphery of the first core, having a refractive index n₂ (<n₁); and athird core, provided on the outer periphery of the second core, having arefractive index n₃ (>n₂, <n₁). Also, the cladding region comprises aninner cladding, provided on the outer periphery of the third core,having a refractive index n₅ (<n₃); and an outer cladding, provided onthe outer periphery of the inner cladding, having a refractive index n₄(<n₃, >n₅); whereas the inner and outer claddings constitute thedepressed cladding structure.

In the optical fiber according to the twentieth embodiment, the outsidediameter 2 a of the first core is 5.6 μm, the outside diameter 2 b ofthe second core is 19.7 μm, the outside diameter 2 c of the third coreis 28.1 μm, and the outside diameter 2 d of the inner cladding is 42.0μm. With reference to the refractive index n₄ of the outer cladding, therelative refractive index difference Δ₁ (=(n₁−n₄)/n₄) of the first coreis 0.55%, the relative refractive index difference Δ₂ (=(n₂−n₄)/n₄) ofthe second core is 0.01%, the relative refractive index difference Δ₃(=(n₃−n₄)/n₄) of the third core is 0.16%, and the relative refractiveindex difference Δ₅ (=(n₅−n₄)/n₄) of the inner cladding is −0.05%.

The optical fiber according to the twentieth embodiment has azero-dispersion wavelength at 1.40 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.059 ps/nm²/km,the dispersion slope at a wavelength of1.55 μm is 0.043 ps/nm²/km, andthe cutoff wavelength is 1.59 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−15.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −6.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.7 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.4 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.5 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.001 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 50.4 μm².

Twenty-first Embodiment

The twenty-first embodiment of the optical fiber according to thepresent invention is an optical fiber having a refractive index profilesimilar to that of the above-mentioned fifth embodiment shown in FIG. 6,while having a low dispersion slope.

As in the above-mentioned fifth embodiment, the optical fiber accordingto the twenty-first embodiment comprises a first core having arefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); a third core,provided on the outer periphery of the second core, having a refractiveindex n₃ (>n₂, <n₁); and a cladding region, provided on the outerperiphery of the third core, having a refractive index n₄ (=n₂).

In the optical fiber according to the twenty-first embodiment, theoutside diameter 2 a of the first core is 6.1 μm, the outside diameter 2b of the second core is 17.8 μm, and the outside diameter 2 c of thethird core is 25.4 μm. With reference to the refractive index n₄ of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.45%, the relative refractive indexdifference of the second core is 0% since it is set such that n₂ =n₄,and the relative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of thethird core is 0.14%.

The optical fiber according to the twenty-first embodiment has azero-dispersion wavelength at 1.40 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.057 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.046 ps/nm²/km, andthe cutoff wavelength is 1.44 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−15.2 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −6.5ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.7 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.5 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.8 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.1 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 64.4 μm².

Twenty-second Embodiment

As in the sixth embodiment shown in FIG. 7, the twenty-second embodimentof the optical fiber according to the present invention has a refractiveindex profile of a depressed cladding structure, while having a lowdispersion slope. In the refractive index profile of the twenty-secondembodiment, contrary to the above-mentioned twentieth embodiment, therefractive index (n₂) of the second core is set lower than therefractive index (n₄) of the cladding region.

In the optical fiber according to the twenty-second embodiment, as inthe above-mentioned sixth embodiment, the core region comprises a firstcore having a refractive index n₁; a second core, provided on the outerperiphery of the first core, having a refractive index n₂ (<n₁); and athird core, provided on the outer periphery of the second core, having arefractive index n₃ (>n₂, <n₁). Also, the cladding region comprises aninner cladding, provided on the outer periphery of the third core,having a refractive index n₅ (<n₃); and an outer cladding, provided onthe outer periphery of the inner cladding, having a refractive index n₄(<n₃, >n₅); whereas the inner and outer claddings constitute thedepressed cladding structure.

In the optical fiber according to the twenty-second embodiment, theoutside diameter 2 a of the first core is 6.0 μm, the outside diameter 2b of the second core is 19.7 μm, the outside diameter 2 c of the thirdcore is 30.0 μm, and the outside diameter 2 d of the inner cladding is44.8 μm. With reference to the refractive index n₄ of the outercladding, the relative refractive index difference Δ₁ (=(n₁−n₄)/n₄) ofthe first core is 0.46%, the relative refractive index difference Δ₂(=(n₂−n₄)/n₄) of the second core is −0.05%, the relative refractiveindex difference Δ₃ (=(n₃−n₄)/n₄) of the third core is 0.16%, and therelative refractive index difference Δ₅ (=(n₅−n₄)/n₄) of the innercladding is −0.05%.

The optical fiber according to the twenty-second embodiment has azero-dispersion wavelength at 1.39 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.052 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.023 ps/nm²/km, andthe cutoff wavelength is 1.66 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−14.4 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −5.7ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.8 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 5.9 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 7.0 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.3 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 55.6 μ².

FIG. 11 is a table listing various characteristics of respective opticalfibers according to the above-mentioned first to thirteenth embodiments.Also, FIG. 12 is a table listing various characteristics of respectiveoptical fibers according to the above-mentioned fourteenth totwenty-second embodiments. As shown in these tables, each of the opticalfibers according to the first to twenty-second embodiments has only onezero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60μm, whereas this zero-dispersion wavelength lies within a wavelengthrange of 1.37 μm to 1.50 μm. In particular, the zero-dispersionwavelength lies within a wavelength range of 1.37 μm to 1.43 μm in thethird, fourth, sixth to ninth, eleventh to fourteenth, and seventeenthto twenty-second embodiments, whereas it lies within a wavelength rangeof longer than 1.45 μm but not longer than 1.55 μm in the second, fifth,fifteenth, and sixteenth embodiments. In each of the embodiments, theabsolute value of dispersion slope at the zero-dispersion wavelength is0.10 ps/nm²/km or less, whereas the cutoff wavelength is 1.3 μm orshorter. Therefore, each of these optical fibers is of a single modehaving no zero-dispersion wavelength at the 1.3-μm wavelength band nor1.5-μm wavelength band, while the dispersion at each of these wavelengthbands is kept low, thereby being suitable for optical communicationsutilizing a plurality of wavelength bands. At a wavelength of 1.55 μm,the first, second, sixth, thirteenth to fifteenth, and eighteenth totwenty-second embodiments have a dispersion slope of 0.06 ps/nm²/km,with the eighteenth to twenty-second embodiments having a further lowerdispersion slope in particular.

Also, in each of the optical fibers according to the first totwenty-second embodiments, the dispersion slope monotonously changes ina wavelength range of 1.30 μm to 1.55 μm, whereas the absolution valueof dispersion at wavelengths of 1.3 μm and 1.55 μm is 12 ps/nm/km orless. Therefore, the absolute value of dispersion in the 1.3-μmwavelength band and 1.55-μm wavelength band in each of these opticalfibers is sufficiently smaller than the dispersion value (about 17ps/nm/km) in the 1.55-μm wavelength band of a conventional standardsingle-mode optical fiber having a zero-dispersion wavelength in the1.3-μm wavelength band. If the dispersion value up to that (about 17ps/nm/km) in the 1.55-μm wavelength band of the above-mentioned standardsingle-mode optical fiber is permissible in an optical transmissionsystem as a whole, then each of the respective optical fibers accordingto the first to twenty-second embodiments is suitably utilized inoptical communications having a signal light wavelength band within arange of 1.2 μm to 1.7 μm.

Further, each of the optical fibers according to the first totwenty-second embodiments has a bending loss of 0.5 dB or less per turnat a wavelength of 1.55 μm when wound at a diameter of 32 mm, with thisbending loss being 0.06 dB or less in the first to sixth, eleventh,thirteenth to seventeenth, nineteenth, and twentieth embodiments inparticular, and thus is preferable in that it can effectively suppressthe increase in loss caused by cabling and the like. Also, each of theoptical fibers according to the first to twenty-second embodiments hasan effective area A_(eff) of 45 μm² or more at a wavelength of 1.55 μm,with the effective area A_(eff) in the first, third to fourteenth, andsixteenth to twenty-second embodiments exceeding 49 μm² in particular,which is on a par with or greater than the effective area ofconventional dispersion-shifted optical fibers. Hence, the lightpropagating through the optical fiber has a lower intensity per unitcross-sectional area, whereby nonlinear optical phenomena such asfour-wave mixing can effectively be suppressed.

In the refractive index profiles 150 to 950 shown in FIGS. 2B, and 3 to10, the maximum and minimum values of relative refractive indexdifference with reference to the refractive index of the referenceregion (the cladding region 120, or the outer cladding if the claddingregion 120 has a depressed cladding structure) of pure silica (silicawhich is not intentionally doped with impurities) is 1% or less and−0.5% or more, respectively, except for the above-mentioned thirteenthembodiment. Though the thirteenth embodiment comprises a structure inwhich the cladding region 120 is doped with fluorine so as to relativelyenhance the difference in refractive index between the core region madeof pure silica and the cladding region, the maximum value of relativerefractive index difference of the core region 110 with respect to thecladding region 120 is 1% or less even in this embodiment. While a highrefractive index region is realized by doping with Ge element, forexample; since its relative refractive index difference is 1% or less,the making of this optical fiber (refractive index control by dopingwith impurities) is relatively easy, and its transmission loss becomessmaller. While a low refractive index region, on the other hand, isrealized by doping with F element, for example; since its relativerefractive index difference is −0.5% or more, the making of this opticalfiber is easy in this regard as well.

FIG. 13 is a graph showing a dispersion characteristic of the opticalfiber according to the first embodiment with respect to wavelength. Asshown in this graph, the dispersion slope monotonously increases in awavelength range of 1.30 μm to 1.55 μm. Also, FIGS. 14 and 15 are graphsshowing transmission loss characteristics with respect to wavelength ofthe optical fiber according to the first embodiment in cases wheredehydration processing is insufficient and sufficient, respectively. Asshown in these graphs, an increase in transmission loss caused by OHabsorption is seen at a wavelength of 1.38 μm. In an optical fiberhaving such a transmission loss characteristic as that shown in FIG. 14,the dehydration processing is not sufficiently effected, so that the OHgroup content is large, whereby the amount of increase in transmissionloss caused by OH absorption is about 0.5 dB/km. An optical fiber havingsuch a transmission loss characteristics as that shown in FIG. 15, onthe other hand, the dehydration processing is sufficiently effected soas to reduce the OH group content, whereby the increase in transmissionloss caused by OH absorption is suppressed to about 0.01 dB/km. When theabove-mentioned wavelength band is utilized as a signal wavelength band,the zero-dispersion wavelength can be set within a range of longer than1.45 μm but not longer than 1.55 μm. The same holds true for therespective dispersion characteristics and transmission characteristicswith respect to wavelength of the optical fibers according to the secondto twelfth and fourteenth to twenty-second embodiments.

Also, FIG. 16 is a graph showing a transmission loss characteristic withrespect to wavelength of the optical fiber according to the thirteenthembodiment in the case where the dehydration processing is insufficient.In the thirteenth embodiment, the increase in transmission loss causedby OH absorption (at a wavelength of 1.38 μm) is 0.3 dB/km when thedehydration processing is not sufficiently effected. If the dehydrationprocessing is sufficiently effected, however, then the increase intransmission loss at a wavelength of 1.3 μm (at a wavelength of 1.38 μm)can be suppressed to 0.01 dB/km or less, as shown in FIG. 14, also inthe case of the thirteenth embodiment.

Without being restricted to the above-mentioned individual embodiments,the optical fiber according to the present invention can be modified invarious manners; and, for example, other designs are possible within thescope of the present invention.

FIG. 17A is a graph showing relationships between effective area A_(eff)and dispersion slope at a wavelength of 1.55 μm mainly concerning theeighteenth to twenty-second embodiments. In this graph, P1, P5, P7, P9,P10, and P18 to P22 are points indicating the relationships betweeneffective area A_(eff) and dispersion slope in the first, fifth,seventh, ninth, tenth, and eighteenth to twenty-second embodiments,respectively.

As can also be seen from this graph, the dispersion slope at awavelength of 1.55 μm can particularly be lowered in the case of opticalfibers (eighteenth to twenty-second embodiments) having such arefractive index profile as that shown in FIG. 6. Also, the effectivearea A_(eff) at a wavelength of 1.55 μm in the optical fibers accordingto the eighteenth to twenty-second embodiments is greater than 49 μm².

Further, FIG. 17B is a graph showing relationships between cutoffwavelength λc and bending loss per turn when bent at a diameter of 32 mmat a wavelength of 1.55 μm concerning main embodiments. In this graph,P1, P3, P4, P6, P7, P10, and P14 to P16 show the relationships betweencutoff wavelength λc and bending loss in the first, third, fourth,sixth, seventh, tenth, and fourteenth to sixteenth embodiments,respectively. Also, the hatched portion in this graph is an area inwhich points indicating relationships between cutoff wavelength λc andbending loss are intensively plotted with regard to conventional opticalfibers having a refractive index profile similar to that shown in FIG.6. Therefore, for avoiding this area (hatched portion), i.e., foryielding a bending loss of 1.0 dB/turn or less, preferably 0.06 dB/turnor less at 32 mm at a wavelength of 1.55 μm, it is preferred that thecutoff wavelength λc be 1.05 μm or more, more preferably 1.3 μm or more.

Embodiments of the optical transmission system according to the presentinvention will now be explained. FIG. 18A is a view showing a schematicconfiguration of an embodiment of the optical transmission systemaccording to the present invention. The optical transmission systemshown in this drawing comprises transmitters 11, 12; opticaltransmission lines 21, 22; a multiplexer 30; an optical fiber 40; ademultiplexer 50; optical transmission lines 61, 62; and receivers 71,72.

The transmitter 11 outputs signal light (first signal light) in the1.3-μm wavelength band; whereas the optical transmission line 21 is atransmission medium for guiding the signal light in the 1.3-μmwavelength band outputted from the transmitter 11 to the multiplexer 30and, for example, is a standard single-mode optical fiber having azero-dispersion wavelength in the 1.3-μm wavelength band. Thetransmitter 12 outputs signal light (second signal light) in the 1.55-μmwavelength band; whereas the optical transmission line 22 is atransmission medium for guiding the signal light in the 1.55-μmwavelength band outputted from the transmitter 12 to the multiplexer 30and, for example, is a dispersion-shifted optical fiber having azero-dispersion wavelength in the 1.55-μm wavelength band.

The multiplexer 30 multiplexes the signal light in the 1.3-μm wavelengthband and signal light in the 1.55-μm wavelength band propagated throughthe optical transmission lines 21, 22, and outputs thus multiplexedlight to the optical fiber 40. The optical fiber 40 transmits the signallight in the 1.3-μm wavelength band and signal light in the 1.55-μmwavelength band multiplexed by the multiplexer 30 toward thedemultiplexer 50. The demultiplexer 50 demultiplexes the signal light inthe 1.3-μm wavelength band and signal light in the 1.55-μm wavelengthband propagated through the optical fiber 40.

The above-mentioned optical fiber 40 is an optical fiber according tothe present invention having a configuration mentioned above, in whichonly one zero-dispersion wavelength exists within a wavelength range of1.20 μm to 1.60 μm, whereas this zero-dispersion wavelength lies withina wavelength range of 1.37 μm to 1.50 μm (preferably within a wavelengthrange of 1.37 μm to 1.43 μm or within a wavelength range of longer than1.45 μm but not longer than 1.50 μm). Also, in the optical fiber 40, theabsolute value of dispersion slope at the zero-dispersion wavelength is0.10 ps/nm²/km or less (preferably 0.06 ps/nm²/km or less at awavelength of 1.55 μm). In a more preferred embodiment, the opticalfiber 40 has a dispersion slope monotonously changing in a wavelengthrange of 1.30 μm to 1.55 μm, whereas each of the absolute values ofdispersion at wavelengths of 1.3 μm and 1.55 μm is 12 ps/nm/km or less,the bending loss at a wavelength of 1.55 μm when wound at a diameter of32 mm is 0.5 dB or less (preferably 0.06 dB or less) per turn, theeffective area A_(eff) at the wavelength of 1.55 μm is 45 μm² or more(greater than 49 μm²), or the increase in transmission loss caused by OHabsorption at a wavelength of 1.38 μm is 0.1 dB/km or less.

The optical transmission line 61 is a transmission medium for guidingthe signal light in the 1.3-μm wavelength band demultiplexed by thedemultiplexer 50 to the receiver 71 and, for example, is a standardsingle-mode optical fiber having a zero-dispersion wavelength in the1.3-μm wavelength band. The receiver 71 receives the signal light in the1.3-μm wavelength band propagated through the optical transmission line61. On the other hand, the optical transmission line 62 is atransmission medium for guiding the signal light in the 1.55-μmwavelength band demultiplexed by the demultiplexer 50 to the receiver 72and, for example, is a dispersion-shifted optical fiber having azero-dispersion wavelength in the 1.55-μm wavelength band. The receiver72 receives the signal light in the 1.55-μm wavelength band propagatedthrough the optical transmission line 62.

In the optical transmission system according to this embodiment, thesignal light in the 1.3-μm wavelength band having arrived at themultiplexer 30 by way of the optical transmission line 21 after beingoutputted from the transmitter 11, and the signal light in the 1.55-μmwavelength band having arrived at the multiplexer 30 by way of theoptical transmission line 22 after being outputted from the transmitter12 are multiplexed by the multiplexer 30, and thus multiplexed lightpropagates through the optical fiber 40 and reaches the demultiplexer50. The multiplexed light having arrived at the demultiplexer 50 isdemultiplexed thereby into the signal light in the 1.3-μm wavelengthband and the signal light in the 1.55-μm wavelength band. Thedemultiplexed signal light in the 1.3-μm wavelength band reaches thereceiver 71 by way of the optical transmission line 61, whereas thesignal light in the 1.55-μm wavelength band reaches the receiver 72 byway of the optical transmission line 62.

Thus, the optical fiber 40 used in the optical transmission system ofthis embodiment comprises a structure which realizes favorable opticalcommunications in both of the 1.3-μm wavelength band and 1.55-μmwavelength band, whereby the optical transmission system employing theoptical fiber 40 enables large-capacity communications.

Without being restricted to the above-mentioned embodiment, the opticalfiber according to the present invention can be modified in variousmanners. For example, the optical fiber, which is a transmission medium,disposed between the multiplexer 30 and demultiplexer 50 may beconstituted by a plurality of optical fibers 40 a to 40 c as shown inFIG. 18B.

According to the present invention, as explained in the foregoing, theoptical fiber has only one zero-dispersion wavelength within awavelength range of 1.37 μm to 1.50 μm including a wavelength of 1.38 μmat which an increase in transmission loss caused by OH absorption isseen, preferably within a wavelength range of 1.37 μm to 1.43 μm, orwithin a wavelength range of longer than 1.45 μm but not longer than1.50 μm, whereas no zero-dispersion wavelength exists in the vicinity ofthe 1.3-μm wavelength band and 1.55-μm wavelength band sandwiching thesewavelength ranges. Therefore, when these wavelength bands are utilizedas a signal light wavelength band, dispersion is intentionallygenerated, so as to effectively suppress nonlinear optical phenomenasuch as four-wave mixing. Also, since the absolute value of dispersionslope at thus set zero-dispersion wavelength is 0.10 ps/nm²/km or less(preferably 0.06 ps/nm²/km or less at a wavelength of 1.55 μm), therespective dispersions in the 1.3-μm wavelength band and 1.55-μmwavelength band are homogenized. When such an optical fiber is employedin the transmission line of the optical transmission system, favorableoptical communications are possible in both of the 1.3-μmwavelength bandand 1.55-μm wavelength band.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

What is claimed is:
 1. An optical fiber, having: only onezero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60μm, said zero-dispersion wavelength existing within a wavelength rangeof 1.37 μm to 1.50 μm; a positive dispersion slope at saidzero-dispersion wavelength; and an increase in transmission loss, whichis caused by OH absorption at a wavelength of 1.38 μm, of 0.1 dB/km orless.
 2. An optical fiber according to claim 1, wherein said opticalfiber has a bending loss which becomes 0.06 dB/turn or less at awavelength of 1.55 μm when wound at a diameter of 32 mm.
 3. An opticalfiber according to claim 1, wherein said optical fiber has a cutoffwavelength of 1.05 μm or more.
 4. An optical fiber according to claim 1,wherein said zero-dispersion wavelength exists within a wavelength rangeof longer than 1.45 μm but not longer than 1.50 μm.
 5. An opticaltransmission system, comprsing: a transmitter for transmitting signalsof 1.38 μm-wave-length band; an optical fiber according to claim 1through which said signals of 1.38 μm-wavelength band propagate; and areceiver for receiving said signals of 1.38 μm-wavelength band from saidoptical fiber.
 6. An optical transmission system according to claim 5,wherein at least one of said signals 1.38 μm-wavelength band has awavelength of 1.37 μm or more but 1.43 μm or less.
 7. An opticaltransmission system according to claim 5, wherein signals of 1.55μm-wavelength band propagate through said optical fiber together withsaid signals of 1.38 μm-wavelength band.
 8. An optical transmissionsystem according to claim 7, wherein signals of 1.3 μm-wavelength bandpropagate through said optical fiber together with said signals of 1.55μm-wavelength band.